Optical techniques have been used for some years for the detection and classification of particles in liquids. The concentration of particles in a liquid can be measured using a turbidimeter. A beam of light is transmitted through a liquid sample and attenuation of the light received at a detector provides a measure of the concentration of particles in the sample. Reference is made to Blackburn C de W, et al, (1987) “Brief Evaluation of a Fully Automated Optical Analyser System, the Bioscreen for Measuring the Growth of Micro-organism”. Technical Notes No. 57, Leatherhead Food R. A.
It is also known to monitor light scattered from particles in a liquid sample. The total amount of light scattered from an illuminated volume of sample and received at a detector can provide an indication of the concentration of scattering centres or particles in the sample. This technique is called nephelometry and has been used for immunoassay reaction detection.
Techniques have also been described using coherent light from a laser and monitoring the speckle pattern produced in the light scattered from particles in the sample. Both the effect of interference between light from different scattering centres in the sample due to different path lengths to the detector, and the effect of small Doppler changes in wavelengths of scattered light due to particle motion, have been investigated. U.S. Pat. No. 4,826,319 and U.S. Pat. No. 6,011,621 describe measuring or monitoring the size of particles in a liquid by observing the frequency content of the scattered light.
It has also been proposed to detect living micro-organisms in liquid samples by observing the characteristic frequencies of intensity fluctuations of scattered light-resulting from the motility of the organisms. See, for example, “Spectral Analysis of Laser Light Scattered from Motile Micro-organisms”, by R. Nossal, Biophysical Journal Vol. 11 (1971), pp341–354. However, there are substantial technical problems in using such techniques for practical purposes, especially for samples with low concentrations of micro-organisms or other particulates.
For the theory and a mathematical analysis of coherent light scattering from particles suspended in a liquid, reference should be made to “Photon Correlation and Light Beating Spectroscopy” edited by H. Z. Cummins and E. R. Pike, Plenum Press, 1974, especially pages ?–?.